I have also used the Keystone blocks. I put up about 250 LF 3' high wall and 100 lf around the island in my pond. I used LaborFinder for strong backs. ~$9 per hour 4 hr min.
Kubota B7100, JD Gator 6X4, Kubota L2350, JD 14T Baler, JD 894A Side Delivery Rake, IH 100 Balance Sickle Mower
Jinman, I don't want to sound negative, but the fellow I bought my property from used those same type of timbers for fence posts, and they only lasted a few years, much less time then normal fence posts. I have had to replace several already ( in 2 years ), they rot off below ground level.
I think when they trim the telephone/utility poles down to 6x6 they cut a lot of the preservative soaked wood off.
I hope you have better luck!
Oh, I also replaced a good bit of railroad tie retaining walls with some Anchor retaining wall interlocking stones. The railroad ties were rotting and infested with bugs, possibly even termites. The Anchor blocks work very well.